Various bodies worldwide are currently developing standards for the specification of the next generation of mobile telecommunications systems. Services offered by current wireless mobile systems are telephony and voice services supported by narrowband digital networks. There will be a demand for higher bandwidth services as more comprehensive data and information is transmitted. This comprehensive data will require mobile systems to interface with hardwired broadband networks using asynchronous transfer mode (ATM) transmission (defined below). Thus, today's wireless interface must carry narrowband services effectively while providing the flexibility to carry higher bandwidth services as the demand increases.
However, the harmonization of multiple communication services with different characteristics results in distinguishable spectrum and transmission needs. Representative services used on wireless communication networks include telephony, videotelephony, and high-speed data transmission. These services have varying and distinguishable needs which include being in high demand, being delay critical, requiring high bandwidth, and/or being intolerant of errors. These different services also have different encoding requirements, different error transmission requirements and different delay requirements. The trade-offs of these different requirements of the different services used on the network, when they are integrated into a single cohesive whole, lead to limitations in the ability of the network to transmit a large amount of information quickly, correctly and simultaneously.
The radio access technique most often utilized for these diverse requirements is known as code division multiple access (CDMA). CDMA and ATM characteristics, separately and in combination, offer significant advantages in wireless communication environments where a wide range of services must be carried. Both CDMA and ATM allow a transmission link to support a number of simultaneous connections which can be used on demand to simplify routing and reduce traffic congestion and overhead.
CDMA allows many users to share the same radio frequency spectrum simultaneously through the use of spread spectrum transmission. Each individual connection across the radio interface is distinguished by a CDMA code allocated to that connection. Since there is a relatively large number of codes, they can be allocated to new connections as the connections are set up or when a new mobile station affiliates to a base station servicing multiple users. User data is transmitted over the air interface with an associated CDMA code and without the need for additional channel assignments. Thus, the CDMA code identifies the signal and represents a "virtual" channel connection for the air interface.
A reality of wireless communications is that data is communicated at essentially random times. Additional data may be added to the system and transmitted at any time. These random transmissions may, in the aggregate, force the system capacity to be exceeded and cause interference between users. These dynamically changing traffic characteristics may increase above system limitations and cause unacceptably excessive error rates.
ATM subdivides data for transmission into small fixed size packets called ATM cells which contain groups of information. Each ATM cell includes a data field and a control field which includes an address. The address within the control field can also be considered a virtual channel connection within a fixed network since multiple users are each identified by a separate address allocation. ATM is unlike traditional transmission systems in that it is asynchronous, and only uses network capacity when there is data to be transmitted.
Another communications transfer mode known in the art is time division multiple access (TDMA). TDMA is similar to ATM, with the exception that TDMA is not asynchronous. Each TDMA transmitter sends a "cell" of information each time it is "polled."
In mobile digital information transmission techniques, specifically CDMA, ATM and TDMA, data information is considered to be "bursty" in that significant amounts of data are reduced to "packets" and transmitted in "bursts." Burst mode transmission results in information packages being sent and packetization delays. The process of filling ATM cells with speech also involves packetization delays.
The inherent nature of radio communications, in terms of transmitter power constraints and limited spectrum availability, also restricts the maximum amount of information which is possible to be transmitted over an air interface. Thus, within an air interface, broadband communication services must be regarded as being similar to narrowband services due to the mobile power constraints and the limitations of the data transfer rate on the air interface network. Additionally, radio transmission is significantly more error prone than broadband hard-wired networks. This tends to further reduce capacity due to the necessity to transmit and process error control protocols.
The standard known in the art which was created by the International Telecommunications Union (ITU) for the wireless multimedia communications environment is known as IMT-2000. FIG. 1 shows a graphical depiction of the various subsystems of a mobile radio station in conjunction with an associated base station within a multiple access environment under the IMT-2000 standard.
In FIG. 1, the base station 12 includes a base station control 11 which controls the base station 12. The base station 12 communicates over a wireless interface 13 to a mobile station 14. The mobile station also includes a mobile station control 15. Each of the systems graphically shown in FIG. 1 includes the following subsystems, an internal network 16, link access control subsystems 17, medium access control subsystems 18, and the physical radio air interface transmission system 19.
Current wireless communication of data, as used and as planned for implementation with IMT-2000, uses a system of error correction and reliability known as "Automatic Repeat Request" (ARQ). ARQ is a strategy of error correction which requests the re-transmission of a packet of data when the transmission is not completely and accurately received.
In ARQ, the receiver provides a signal to the corresponding data packet transmitter that the information data packet was not adequately received. Upon receipt of the ARQ signal, signifying an error in the previous transmission, the transmitter again re-transmits the data packet to the receiver. This process is reiterated until the data packet is adequately received. The receiver is then able to receive the next data packet to be sent.
The ARQ process causes system delays as identical information is transmitted and retransmitted over and over again until the signal representing the data packet is deemed acceptable or is considered to have failed and the transmission of that information data packet is aborted. These retransmissions of identical information add unwanted network traffic causing system degradation and interference.
A transmission standard related to CDMA digital wireless communications is Telecommunications Industry Association (TIA) committee TR45.5 developing IS-95 third generation standards. This telecommunications industry standard recommends that "Turbo coding" be provided for data transmission rates higher than 14.4 Kbps.
A known implementation of Turbo encoding is shown in FIG. 4. In the illustrated transmission system, data to be transmitted is input to the encoder 40 at input 41. The data is then processed by a first recursive systematic convolutional coder (RSCC) 42 to provide multiple encoder outputs 43. Each of the set of first encoder outputs 43 is a signal having separately redundant information representative of the data at input 41.
The encoder 40 additionally processes the data at input 41 by sending it to a second RSCC 44 through interleaver 45. Interleaver 45 reshuffles the bits of information at input 41 and sends them to second RSCC 44 in order to minimize the error within the transmission. By reshuffling the data bits, the second set of encoder outputs 46 from second RSCC 44 are in a different order and configuration than at the outputs of first RSCC 42.
In "Turbo coding," the various outputs 43, 46 of the first and second RSCC are then "punctured" or taken from the remainder of the outputs 43, 46 and selected for transmission. Puncturing outputs is acceptable for transmission purposes because of the redundancy of information which is created within the encoder 40. While not described in detail herein, Turbo decoding is also well known in the art.
TIA standard TR45.5 defines several turbo coding rates for use in wireless data communications. Turbo coding rates are a ratio of the information bits being considered over the actual bits being transmitted. These rates are 1/2 or 1/3 which is derived from a standard rate 1/3 code for the forward link, i.e., a transmission of the base station to the mobile station, and 1/2 or 1/3 or 1/4 derived from a standard rate 1/6 code for the reverse link, i.e., a transmission of the mobile station to the base station.
A higher turbo rate code is beneficial for transmission because less bits are actually being transmitted to convey the same amount of information. By "puncturing" some of the outputs 43, 46, a higher rate code is achieved since less bits are actually transmitted and therefore less hardware is required. Due to the redundancy of information being output from the Turbo encoder, puncturing is acceptable for sending a reliable signal without losing information.
One system architecture utilized in wireless spread spectrum communications is known as pre-combining "rake." In this rake architecture, multiple path parameters for the received signal are derived from a downlink pilot signal and used for phase, amplitude, and time alignment of the various multiple path components which are combined prior to demodulation. Essentially, pre-combining rake recognizes that a single transmitted signal sent over a wireless communications link will have multiple components or "metrics" which must be combined by a receiver to obtain a single accumulated signal input for the Viterbi Algorithm, known in the art.
When the rake architecture is used within wireless communications systems, a transmission is deemed to fail when insufficient multiple path components are received and their combined power level is not above a predetermined threshold.
When the transmission fails, the rake receiver must undertake an error correction system. It transmits an ARQ request. The information signal is then re-transmitted by the transmitter. This process is reiterated until the signal is deemed to be adequately received, i.e., it is above the predetermined power threshold when rake recombination is completed.
However, the addition of multiple transmissions of the same signal adds unwanted traffic within the transmission network. Further, the rake system has power constraints due to multiple users on the network with different power availability. One user's transmission may interfere with the power level of another user's reception.